Air quality in the coastal city of Chennai in the past two months was consistently poor with all the samples designated as at least 'unhealthy' as per US standards, an international expert said on Sunday.

Levels of toxic metal manganese in the air during this period averaged 55 per cent higher than they were averaged in April 2017, whereas lead levels averaged nearly three times higher in two months than they were averaged in April.

Mark Chernaik, a scientist associated with the Environmental Law Alliance Worldwide who studied the reports, told IANS that five of the samples in Chennai in the past two months reached a level of 'very unhealthy' and two of them 'hazardous' under the US Environmental Protection Agency (EPA) Air Quality Index.

All of the 12 samples substantially exceeded the Indian 24-hour National Ambient Air Quality Standard for PM (tiny particulate matter) of 60 micrograms (Aug)/m3.

According to Chernaik, the levels of PM 2.5 (a crucial indicator of dangerous pollution) in December and January averaged 165 Aug/m3, 35 per cent higher than PM 2.5 averaged in April.

High levels of manganese, a neurotoxicant, excess expose causes adverse neurobehavioural health effects. It averaged 0.16 Aug/m3 in two months. The World Health Organisation prescribes an annual average standard of 15 Ig/m3.

Likewise, exposure to another neurotoxicant lead causes irreversible mental retardation in children. It averaged 0.16 Aug/m3 in the two months, exceeding the US Environmental Protection Agency's National Ambient Air Quality standard for lead of 0.15 Aug/m3 on a three-month rolling basis.

Chernaik says the air quality in Chennai seems heavily impacted by re-suspension of dust.

"The emissions from the paved road dust contribute significantly to the pollution load as far as PM 10 is concerned. The contribution from the vehicular sources towards PM is only around 10 per cent in Chennai," he said.

The recommendations for policy to be adopted for meeting the pollution levels are: Sweeping and watering of the paved roads to reduce the emissions of the particulate matter.

Aluminum, iron, calcium and silicon, enriched in coal ash are indicators of pollution caused by re-suspended dust. Levels of aluminum, iron, calcium and silicon combined constituted roughly 15 per cent of PM 2.5 in ambient air samples collected both in December and January and in April 2017.

Levels of PM 2.5 trended higher on days when the percentage of combined levels of aluminum, iron, calcium and silicon were higher.

Manganese and lead are also constituents of coal ash.

It is reasonable to assume that over several decades, manganese and lead in coal ash disposed of in Chennai has permeated the city and is re-suspended on a daily basis from paved roads, he added.

Shweta Narayan of Healthy Energy Initiative, India, who is based in Chennai, told IANS: "Considering that aluminum, iron, calcium and silicon that are enriched in coal ash are also enriched in the Chennai dust samples, and given the cluster of coal-fired thermal plants in Ennore, it is reasonable to assume that coal ash dust is a significant contributor to Chennai's air pollution."

A total of 1,189 sq km Chennai Metropolitan Area currently has real-time air quality monitors that measure the levels of PM 2.5 at only four locations.